The Food and Drug Administration today announced that a recently discovered so-called "undetectable steroid" is an illegal drug, not a dietary supplement.

As a result, the substance known as tetrahydrogestrinone, or THG, cannot be legally marketed without FDA approval under the agency's rigorous approval standards that are meant to ensure that drugs sold to Americans are safe and effective.

The decision comes after last week's U.S. Anti-Doping Agency investigation into a Burlingame-based laboratory that was identified as an alleged source of THG.

Today, FDA officials said they are concerned about the marketing and use of the unapproved product and the agency is working with other federal law enforcement agencies to aggressively "engage, enforce and prosecute those firms or individuals who manufacture, distribute or market THG."

This could include Burlingame's BALCO Labs. Several high-profile Bay Area athletes, including Giants slugger Barry Bonds, have been called upon to testify in the case against BALCO.

"Our mission is to protect the American public from this potentially harmful product," said FDA Associate Commissioner for Regulatory Affairs John Taylor.

In the meantime, the FDA is warning consumers that while little is known about the safety of the drug, its structure and relationship to better known products had led the agency to believe that its use may pose considerable risks to individuals' health.

FDA officials say that while suppliers of THG may represent it as a dietary supplement, it doesn't meet the dietary supplement definition. Instead, they say, it is a purely synthetic "designer" steroid derived by chemical modification from another anabolic steroid that is explicitly banned by the U.S. Anti-Doping Agency.

Federal health officials are warning users of the substance's potential health effects because of its close structural relationship to anabolic steroids, which have been known to not only build muscle mass, but also to cause serious, long-term health consequences in men, women and children, including liver damage, heart disease and anxiety.